[core] remove redundant identity function

Dry::Transformer::Coercions provide this function OOTB

Author: solnic

core/lib/rom/processor/transformer.rb

@@ -28,10 +28,6 @@ module Functions
 
         INVALID_INJECT_UNION_VALUE = '%s attribute: block is required for :from with union value.'
 
-        def self.identity(tuple)
-          tuple
-        end
-
         def self.get(arr, idx)
           arr[idx]
         end